The present invention relates to dual gain semiconductor amplifiers and more particularly to a method and means for providing low-power, low-noise operation.
The present invention is discussed in the context of a high frequency (e.g. 900 MHz). It is discussed in this context since it provides readily appreciable benefits, but is useful in transceiver applications. One such particular application is radio frequency links in general. One such application is a radio frequency hearing aid product. An example of such a prior art product is found at Deiss, A et al., A 200 MHz Sub-mA RF Front End For Wireless Hearing Aid Applications, IEEE Journal of Solid-State Circuits, Vol. 35 No. 7 July 2000, pp. 977-985. In this article, a hearing aid system with a radio frequency connection between both ear pieces is described. The transceiver in this example has a desired power dissipation of 2-3 mW. The authors report that a 900 MHz radio frequency low-noise amplifier front-end would be preferable. Use of a 900 MHz frequency permits the use of a physically smaller antenna and off-chip components for the ear link. However, the authors also report that it is extremely difficult to have a dual gain low-noise amplifier that only dissipates 2-3 mW at higher frequencies such as 900 MHz and beyond.
It is therefore the object of the present invention to provide an amplifier topology providing for low-power, low-noise operation over a significant range of frequencies useful for RF links.
It is also the object of the present invention to provide a method for efficiently and effectively multiplying a radio frequency signal by a low or high gain factor. It is a particular object of the present invention in one form to provide low-noise dual gain amplifier of the type described that only dissipates 2-3 mW at higher frequencies such as 900 MHz and beyond.
In accordance with the present invention, there is provided a circuit topology in which a first amplifier transistor is connected in the common emitter mode for operation in a high gain mode and a second amplifier is connected in the common collector configuration for operation in a low gain mode low gain amplification. The collector-emitter circuits of the low gain and high gain amplifier transistors are AC coupled in series between a bus voltage and ground. Biasing means connected to the bases of each transistor bias the transistors to opposite states of conductivity. An additional switching means is AC coupled between a radio frequency input signal terminal to one amplifier input and rendered conductive or non-conductive in accordance with the gain mode. According to the method of the present invention, the radio frequency signal is amplified selectively by the low gain or high gain amplifier described above.